Choosing the proper clutch can mean the difference between finding Victory Lane and finding yourself a lap down, or worse, on the trailer. The best clutch for your application isn’t necessarily the lightest, the biggest or smallest, or the least or most expensive. Instead, it is better to study the weight of your race car, tire type and engine type, driveline, horsepower, torque capacity and rules package to determine the clutch you need.
Lightweight applications can utilize lighter, smaller clutches, but that also means that heat will build up quicker. Heavier clutches with more discs on the other hand are more durable and withstand more abuse from heat, but they have a greater moment of inertia. The key is to find the best balance for your application.
Moment of inertia (MOI) is a measurement of how much energy it takes to spin an object, dependent upon the amount and distribution of its mass. For instance, the heavier or larger the clutch in diameter, the harder it is to accelerate or decelerate. If two clutches are the same weight and one is half the diameter of the other, then the smaller diameter clutch will accelerate and decelerate faster with less overall energy being used. This translates to a faster acceleration coming out of a turn, and a faster deceleration going into the next one. Faster acceleration translates into a speed advantage over the competition.
While it’s obvious that a small diameter and low MOI is desired, it is also not the only factor to consider when buying a clutch. A smaller clutch is equal to less radial friction surface and low MOI, but in turn it also is more susceptible to heat buildup and a higher rate of wear. This might be fine in a lightweight, four-cylinder Mini Stock application, but if you drive a light clutch around the pits in a heavier, eight-cylinder Street Stock, you will be operating the engine at a low RPM, which will cause it to stall easily, resulting in a higher rate of wear of the clutch components. A clutch with two or three discs will also be able to handle more horsepower and will absorb less heat, but it also features a higher MOI. If you do not plan on replacing your clutch frequently it is important to strike a balance and consider the best setup for performance, reliability, durability and value.
Torque capacity is another major factor to consider during clutch selection. The torque capacity for a clutch is the clutch’s highest ultimate torque rating, or the maximum torque that can be applied on a continual basis while still maintaining a normally expected fatigue life. To size a peak-torque-capacity clutch for your application, multiply the engine’s peak torque by 1.25 and choose a clutch that has as much or more torque capacity. Then add one extra disc for heat capacity and durability. It is important to choose a clutch with more torque capacity than needed to avoid slippage.
Knowing the rules can be complicated, but sometimes your track or sanctioning body will simplify your options. It is important to always check the rulebooks before choosing a clutch. Most sanctioning bodies indicate the minimum clutch diameter permitted for the friction/driven discs. Some sanctions also specify the number of discs allowed, along with the type of friction material. This is often limited to iron or aluminum options. In some cases the rules may even require a completely stock clutch, or one that is “stock appearing.” If you show up at the track with a trick clutch that falls outside of these parameters there is a good chance you will be going home empty-handed or with a lighter wallet.
In addition, it’s important to ensure that all parts work together. For example, if you are using a stock clutch and buy an aftermarket flywheel for your race car, you need to double check that the flywheel bolts clear the clutch springs. Or if you have a new clutch you’ll want to be sure that it fits properly within the bellhousing. Otherwise you may be out of hundreds of dollars and a lot of aggravation. Again, it is important here to check the rulebooks and be sure that all of these parts are legal as well.
Remember that the best policy in choosing a clutch is to do plenty of research and choose the option that strikes the ideal balance for your race vehicle.
Tips: Installation & Maintenance
Keep your race clutch functioning properly for as long as possible by following these simple steps during the installation and life of the clutch.
Proper Installation. Be sure to read manufacturer’s instructions during installation. In addition to step-by-step directions, this document will also most likely cover the importance of using an alignment tool, how to check for proper fitment and cleaning options. With the time it takes to remove and replace OE components, it is worth the bit of extra time to read the instructions. It will save hours and money.
No Loading Zone. To extend the life of your racing clutch do not use it to load the car on your trailer. The amount of slippage caused by doing so actually creates more wear than an entire night of racing.
Regular Inspections. Check your friction discs, pressure plates and floater plate in regular maintenance intervals to be sure that everything is in spec. Although pressure and floater plates may not wear as quickly as the friction discs, they can still deteriorate. Also, be sure to check clutch fluid levels. Without full attention to every component within the clutch, a competitor may be sidelined prematurely.